We have reconstructed the geologic history of SHR using techniques adapted from seismic sequence stratigraphy and ages determined from biostratigraphic analysis of Leg 204 samples. Between 1.6 and 1.2 Ma, sediments from the abyssal plain were accreted to the margin along a seaward-vergent thrust fault. At ~1.2 Ma, vergence direction changed and Fault F2, a landward-vergent fault overlain by a drag fold (Fold F) thrust deep-sea fan sediments over the recently accreted sediments. This episode of landward vergence lasted to ~0.3 Ma. During this time period, uplift in the accretionary complex shifted toward the northeast, from Anticline A on the eastern flank of the present ridge to Anticline B, which is now buried beneath the sediments of the slope basin east of Hydrate Ridge. This migrating uplift resulted in shifting depocenters and overlapping slope basins. Early to middle Pleistocene strata (Units S.IV, S.III, and S.II) of these basins consist of recycled abyssal plain sediments shed from the bathymetric highs and ponded into the depocenters. The cause of this northeastward shift in uplift while the deformation front migrated west remains enigmatic but may be related to passage of a ridge or seamount on the subducting plate. A second reorganization of the deformation front, to seaward thrusting farther to the west, began sometime prior to 0.3 Ma, probably resulting in reactivation of uplift of the modern Hydrate Ridge in response to sediment underplating and duplexing. Normal faulting in the slope basin sediments at the crest accompanied this phase, which likely continues today.
Because of this history, a wide range of lithologies currently falls within the gas hydrate stability zone at SHR. We note a correlation between BSR amplitude and sediment age, with high-amplitude BSR anomalies correlated with uplifts of the older, lithified, and fractured accreted sediments that were originally deposited on the abyssal plain. Gas to feed gas hydrate formation in these sediments is probably exsolved from pore water as a result of decreased gas solubility due to tectonic uplift. We infer that the gas migrates into the gas hydrate stability zone through pervasive fracture permeability in the accretionary complex and that gas migration and gas hydrate formation in the overlying slope basin sediments is limited to a few discrete permeable horizons or faults.